Screen color picker uses system monitor profile regardless of Preferences settings
When a system monitor profile is installed, GIMP-2.10 uses the system monitor profile in connection with color picking from the screen (but not when color picking from an image opened in GIMP), even when the checkbox in "Edit/Preferences/Color Management" to use the system monitor profile is not checked and the user has chosen a monitor profile other than the system monitor profile, as per the following terminal output, when I've selected a LAB LUT monitor profile in Preferences, and not checked "Try to use the system monitor profile":
gimp_color_transform_new: using babl for 'nec-20171223-g22-t5200-R255-G239-B250-qm-cal-qh-aS.icc' -> 'GIMP built-in sRGB' gimp_color_transform_new: using babl for 'nec-20171223-g22-t5200-R255-G239-B250-qm-cal-qh-aS.icc' -> 'GIMP built-in sRGB'
It seems the only way to get GIMP to not use the system monitor profile when color picking from the screen, is to uninstall the system monitor profile before starting GIMP.
I haven't checked, but I'm guessing the same thing happens in GIMP-2.99. Edit: yes, this affects GIMP-2.99.
This bug might be related to #2230 Edit: hmm, probably not related at all.